Treadwell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Treadwell Observatory clear sky chart from 2005-11-06 to 2024-10-24.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	18% 10% 3 8% 5% 5% 5% 4 5% 7% 32%	101468
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 32%	97312
Transparency	Day	51% 5% 12% 21% 10% 1	101227
	Night	53% 5% 12% 20% 10% 1	97173
Seeing	Day	22% 9% 27% 25% 13% 4	14469
	Night	23% 7% 26% 32% 12% 1	23467

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	18% 18% 6% 14% 7% 4 4 3 3 4 20%	2517
	11th to 20th	Day	15% 14% 6% 14% 5% 4 5% 4 6% 5% 22%	2520
	21st to end	Day	15% 18% 7% 10% 7% 6% 6% 4 4 4 18%	2807
February	1st to 10th	Day	14% 16% 5% 11% 7% 5% 4 3 5% 5% 25%	1710
	11th to 20th	Day	13% 18% 5% 11% 5% 5% 4 4 4 5% 27%	1701
	21st to end	Day	19% 14% 5% 9% 6% 6% 4 3 5% 5% 25%	1570
March	1st to 10th	Day	20% 14% 4 9% 7% 4 3 4 5% 5% 26%	3116
	11th to 20th	Day	13% 16% 5% 11% 6% 5% 4 3 5% 5% 26%	3404
	21st to end	Day	16% 8% 3 7% 6% 4 4 3 4 6% 39%	4088
April	1st to 10th	Day	21% 13% 3 7% 5% 3 4 2 4 5% 32%	2280
	11th to 20th	Day	22% 9% 3 6% 4 4 4 3 4 5% 35%	2254
	21st to end	Day	22% 10% 3 7% 5% 4 5% 3 4 4 32%	2416
May	1st to 10th	Day	21% 9% 3 8% 5% 5% 5% 4 6% 6% 29%	4426
	11th to 20th	Day	19% 7% 3 8% 5% 5% 4 4 5% 7% 32%	4398
	21st to end	Day	16% 7% 3 7% 5% 4 5% 4 6% 8% 37%	4837
June	1st to 10th	Day	22% 8% 3 6% 5% 5% 4 5% 5% 7% 29%	2660
	11th to 20th	Day	14% 6% 2 7% 4 5% 6% 5% 6% 8% 36%	2660
	21st to end	Day	20% 7% 3 6% 5% 5% 5% 5% 7% 7% 30%	2660
July	1st to 10th	Day	13% 6% 3 8% 5% 6% 5% 5% 6% 8% 35%	4440
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 4 7% 9% 36%	4440
	21st to end	Day	12% 5% 2 6% 5% 4 5% 4 5% 8% 45%	4814
August	1st to 10th	Day	14% 6% 3 8% 5% 6% 6% 4 6% 8% 34%	4423
	11th to 20th	Day	16% 6% 2 6% 4 4 5% 4 6% 9% 38%	3966
	21st to end	Day	16% 5% 2 6% 5% 4 5% 4 5% 8% 40%	4180
September	1st to 10th	Day	15% 5% 2 6% 4 4 5% 3 5% 7% 43%	2280
	11th to 20th	Day	13% 5% 2 4 4 4 4 3 4 7% 49%	2251
	21st to end	Day	19% 6% 2 5% 3 4 4 4 5% 6% 42%	2075
October	1st to 10th	Day	22% 8% 2 6% 4 4 4 3 5% 7% 35%	1900
	11th to 20th	Day	23% 10% 4 7% 6% 4 4 3 5% 6% 28%	1900
	21st to end	Day	25% 11% 3 7% 5% 5% 4 3 5% 5% 27%	1823
November	1st to 10th	Day	17% 10% 3 6% 5% 5% 4 3 5% 7% 35%	1671
	11th to 20th	Day	21% 12% 4 9% 5% 5% 5% 3 5% 5% 27%	1563
	21st to end	Day	22% 13% 4 9% 6% 5% 4 3 5% 5% 22%	1520
December	1st to 10th	Day	23% 17% 5% 10% 5% 5% 5% 3 4 4 18%	1520
	11th to 20th	Day	21% 16% 4 9% 6% 4 5% 3 4 5% 24%	1906
	21st to end	Day	27% 20% 4 9% 5% 3 4 2 3 3 20%	2772
January	1st to 10th	Night	18% 18% 6% 11% 5% 5% 4 3 4 5% 23%	4429
	11th to 20th	Night	14% 16% 6% 12% 7% 6% 5% 4 5% 5% 22%	4440
	21st to end	Night	16% 20% 6% 10% 6% 5% 4 2 4 5% 21%	4848
February	1st to 10th	Night	14% 14% 5% 11% 7% 5% 4 3 5% 6% 25%	2470
	11th to 20th	Night	14% 12% 5% 12% 7% 5% 4 4 5% 5% 28%	2365
	21st to end	Night	16% 14% 5% 9% 6% 5% 4 3 5% 5% 29%	1884
March	1st to 10th	Night	21% 12% 4 8% 6% 4 3 3 4 4 31%	3724
	11th to 20th	Night	17% 17% 4 8% 5% 4 4 3 4 6% 27%	3553
	21st to end	Night	15% 10% 3 8% 5% 4 4 3 4 5% 39%	3544
April	1st to 10th	Night	23% 10% 2 6% 5% 4 4 3 4 6% 32%	1900
	11th to 20th	Night	20% 9% 2 7% 5% 4 4 3 5% 6% 36%	1875
	21st to end	Night	21% 10% 2 7% 4 4 4 3 5% 6% 34%	1780
May	1st to 10th	Night	20% 9% 3 8% 6% 4 4 4 5% 8% 30%	2514
	11th to 20th	Night	21% 8% 2 6% 4 4 4 3 4 7% 37%	2495
	21st to end	Night	17% 7% 2 7% 5% 4 5% 4 6% 7% 37%	2738
June	1st to 10th	Night	17% 7% 2 7% 5% 4 5% 4 8% 9% 32%	1520
	11th to 20th	Night	15% 7% 3 6% 4 4 4 5% 5% 6% 41%	1520
	21st to end	Night	19% 8% 2 7% 4 5% 5% 5% 5% 6% 36%	1520
July	1st to 10th	Night	15% 6% 3 8% 5% 6% 5% 5% 7% 7% 35%	2520
	11th to 20th	Night	15% 6% 3 6% 4 4 4 3 6% 8% 42%	2520
	21st to end	Night	12% 6% 2 5% 4 4 5% 4 5% 7% 45%	2739
August	1st to 10th	Night	14% 6% 3 7% 5% 4 5% 4 6% 7% 38%	2506
	11th to 20th	Night	18% 7% 2 5% 3 4 4 3 6% 7% 42%	2827
	21st to end	Night	16% 5% 2 7% 4 4 4 4 5% 6% 43%	3476
September	1st to 10th	Night	12% 5% 2 6% 5% 4 5% 4 5% 7% 45%	1900
	11th to 20th	Night	14% 6% 2 4 4 2 3 3 5% 6% 50%	1928
	21st to end	Night	18% 5% 2 5% 4 4 4 3 4 7% 44%	2102
October	1st to 10th	Night	19% 7% 3 7% 4 4 5% 3 6% 6% 38%	2280
	11th to 20th	Night	21% 9% 3 7% 5% 5% 4 4 4 5% 34%	2280
	21st to end	Night	24% 9% 3 8% 5% 4 4 3 4 5% 30%	2600
November	1st to 10th	Night	21% 9% 3 7% 5% 4 3 3 5% 6% 33%	2407
	11th to 20th	Night	19% 11% 4 9% 6% 4 4 3 5% 5% 31%	2616
	21st to end	Night	21% 13% 4 9% 6% 4 5% 3 4 5% 24%	2660
December	1st to 10th	Night	21% 17% 6% 11% 5% 4 4 4 4 5% 18%	2660
	11th to 20th	Night	23% 15% 4 9% 5% 4 4 2 4 4 26%	3288
	21st to end	Night	29% 20% 4 9% 4 3 3 2 3 4 19%	4884

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	68% 4 11% 11% 6% 1	2518
	11th to 20th	Day	62% 4 11% 16% 6% 1	2520
	21st to end	Day	67% 5% 12% 10% 5%	2807
February	1st to 10th	Day	61% 3 10% 18% 6% 2	1710
	11th to 20th	Day	59% 3 12% 16% 9% 1	1710
	21st to end	Day	61% 3 10% 14% 11% 2	1570
March	1st to 10th	Day	61% 3 9% 17% 9% 1	3119
	11th to 20th	Day	59% 4 11% 15% 10% 2	3404
	21st to end	Day	45% 5% 7% 22% 18% 3	4088
April	1st to 10th	Day	55% 4 7% 19% 12% 3	2280
	11th to 20th	Day	51% 4 6% 22% 15% 2	2254
	21st to end	Day	56% 3 9% 15% 13% 4	2416
May	1st to 10th	Day	52% 6% 8% 22% 10% 2	4440
	11th to 20th	Day	50% 5% 15% 19% 9% 2	4402
	21st to end	Day	45% 5% 12% 22% 14% 1	4837
June	1st to 10th	Day	53% 4 11% 20% 11% 1	2660
	11th to 20th	Day	43% 6% 12% 23% 13% 2	2660
	21st to end	Day	50% 6% 13% 20% 10% 1	2660
July	1st to 10th	Day	44% 6% 14% 24% 11%	4370
	11th to 20th	Day	41% 7% 16% 29% 8%	4365
	21st to end	Day	36% 6% 15% 32% 11%	4661
August	1st to 10th	Day	45% 6% 17% 22% 9%	4409
	11th to 20th	Day	41% 6% 17% 29% 6%	4006
	21st to end	Day	41% 7% 17% 22% 12% 1	4180
September	1st to 10th	Day	38% 6% 14% 28% 13%	2280
	11th to 20th	Day	35% 5% 15% 31% 14% 1	2251
	21st to end	Day	42% 6% 14% 27% 11%	2075
October	1st to 10th	Day	49% 4 11% 25% 11% 1	1900
	11th to 20th	Day	56% 5% 14% 19% 7%	1900
	21st to end	Day	59% 3 13% 18% 7% 1	1823
November	1st to 10th	Day	50% 4 14% 23% 7% 1	1671
	11th to 20th	Day	58% 3 14% 16% 9%	1563
	21st to end	Day	63% 4 13% 13% 5% 1	1520
December	1st to 10th	Day	69% 5% 12% 11% 3	1520
	11th to 20th	Day	63% 4 13% 15% 4	1906
	21st to end	Day	71% 3 7% 14% 4	2772
January	1st to 10th	Night	66% 4 15% 11% 4	4429
	11th to 20th	Night	63% 5% 14% 13% 5% 1	4440
	21st to end	Night	67% 4 12% 11% 6% 1	4848
February	1st to 10th	Night	61% 4 10% 17% 7% 1	2470
	11th to 20th	Night	58% 4 10% 20% 8% 1	2367
	21st to end	Night	56% 4 9% 19% 12% 1	1884
March	1st to 10th	Night	56% 3 10% 18% 10% 3	3734
	11th to 20th	Night	59% 4 11% 16% 9% 2	3554
	21st to end	Night	46% 5% 6% 24% 17% 3	3544
April	1st to 10th	Night	54% 4 8% 17% 15% 2	1900
	11th to 20th	Night	51% 4 6% 21% 16% 2	1875
	21st to end	Night	51% 4 8% 19% 16% 4	1780
May	1st to 10th	Night	53% 5% 11% 20% 9% 3	2520
	11th to 20th	Night	47% 5% 12% 20% 13% 2	2496
	21st to end	Night	46% 5% 12% 19% 17% 2	2739
June	1st to 10th	Night	47% 7% 12% 23% 11%	1520
	11th to 20th	Night	42% 5% 12% 23% 16% 2	1520
	21st to end	Night	47% 7% 13% 19% 15% 1	1520
July	1st to 10th	Night	45% 7% 15% 20% 12% 1	2480
	11th to 20th	Night	39% 6% 16% 31% 8%	2472
	21st to end	Night	37% 7% 17% 25% 14%	2651
August	1st to 10th	Night	44% 6% 15% 26% 8%	2496
	11th to 20th	Night	41% 5% 17% 29% 7%	2853
	21st to end	Night	40% 7% 16% 23% 12% 2	3476
September	1st to 10th	Night	38% 6% 11% 28% 16% 1	1900
	11th to 20th	Night	36% 5% 13% 35% 11% 1	1928
	21st to end	Night	40% 4 13% 30% 12%	2102
October	1st to 10th	Night	46% 5% 12% 24% 12% 1	2280
	11th to 20th	Night	52% 5% 12% 22% 9%	2280
	21st to end	Night	57% 4 13% 18% 8% 1	2600
November	1st to 10th	Night	51% 4 15% 21% 9%	2407
	11th to 20th	Night	56% 4 12% 18% 9% 1	2616
	21st to end	Night	62% 5% 13% 16% 5%	2660
December	1st to 10th	Night	68% 5% 10% 14% 4	2660
	11th to 20th	Night	62% 4 8% 18% 7% 1	3288
	21st to end	Night	71% 4 7% 14% 4	4884

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	27% 9% 27% 29% 5% 2	180
	11th to 20th	Day	24% 11% 35% 22% 6% 2	180
	21st to end	Day	21% 7% 31% 30% 10% 1	209
February	1st to 10th	Day	29% 12% 39% 15% 5%	270
	11th to 20th	Day	25% 14% 37% 22% 2	308
	21st to end	Day	28% 10% 31% 27% 3	382
March	1st to 10th	Day	24% 9% 32% 26% 9%	460
	11th to 20th	Day	23% 13% 34% 23% 6% 1	460
	21st to end	Day	27% 10% 32% 18% 10% 3	506
April	1st to 10th	Day	27% 14% 29% 18% 11% 2	460
	11th to 20th	Day	29% 13% 24% 18% 12% 3	452
	21st to end	Day	27% 10% 31% 20% 10% 2	460
May	1st to 10th	Day	24% 8% 29% 29% 7% 3	463
	11th to 20th	Day	19% 10% 27% 25% 14% 5%	487
	21st to end	Day	18% 9% 31% 25% 13% 4	529
June	1st to 10th	Day	26% 6% 25% 27% 12% 5%	490
	11th to 20th	Day	14% 10% 26% 28% 16% 6%	490
	21st to end	Day	22% 9% 25% 26% 14% 4	490
July	1st to 10th	Day	14% 5% 22% 33% 18% 8%	485
	11th to 20th	Day	14% 5% 26% 30% 20% 5%	484
	21st to end	Day	14% 4 23% 33% 20% 6%	532
August	1st to 10th	Day	14% 5% 21% 30% 23% 7%	486
	11th to 20th	Day	16% 4 17% 28% 25% 10%	479
	21st to end	Day	16% 5% 21% 29% 20% 10%	539
September	1st to 10th	Day	18% 4 25% 22% 19% 12%	490
	11th to 20th	Day	14% 4 19% 22% 24% 17%	490
	21st to end	Day	20% 7% 25% 21% 18% 9%	490
October	1st to 10th	Day	25% 9% 24% 20% 16% 6%	490
	11th to 20th	Day	26% 11% 28% 21% 12% 1	490
	21st to end	Day	30% 9% 24% 25% 12%	412
November	1st to 10th	Day	26% 11% 22% 27% 13% 2	365
	11th to 20th	Day	28% 10% 32% 20% 8% 2	223
	21st to end	Day	26% 15% 23% 26% 7% 2	180
December	1st to 10th	Day	31% 8% 29% 22% 9%	180
	11th to 20th	Day	26% 14% 37% 21% 2 1	180
	21st to end	Day	34% 13% 33% 15% 5% 1	198
January	1st to 10th	Night	28% 9% 29% 28% 6%	760
	11th to 20th	Night	28% 7% 28% 29% 7% 1	760
	21st to end	Night	20% 7% 28% 36% 9% 1	836
February	1st to 10th	Night	21% 9% 33% 30% 6%	760
	11th to 20th	Night	20% 9% 36% 29% 6%	743
	21st to end	Night	25% 8% 29% 33% 5%	628
March	1st to 10th	Night	23% 9% 29% 33% 6%	760
	11th to 20th	Night	25% 8% 32% 28% 6% 1	760
	21st to end	Night	22% 7% 31% 31% 9%	836
April	1st to 10th	Night	27% 9% 27% 29% 7%	760
	11th to 20th	Night	25% 10% 28% 27% 9% 1	751
	21st to end	Night	27% 7% 27% 31% 8% 1	640
May	1st to 10th	Night	25% 5% 26% 34% 8% 2	380
	11th to 20th	Night	25% 5% 26% 32% 12% 1	374
	21st to end	Night	16% 5% 30% 35% 13%	411
June	1st to 10th	Night	17% 4 24% 43% 12% 1	380
	11th to 20th	Night	17% 4 23% 41% 14% 1	380
	21st to end	Night	20% 3 23% 35% 18% 2	380
July	1st to 10th	Night	12% 3 19% 45% 20%	370
	11th to 20th	Night	13% 3 18% 46% 19% 1	368
	21st to end	Night	15% 3 16% 46% 19% 1	416
August	1st to 10th	Night	13% 3 12% 47% 23% 1	378
	11th to 20th	Night	15% 3 13% 37% 28% 3	579
	21st to end	Night	15% 3 18% 39% 22% 2	836
September	1st to 10th	Night	14% 4 21% 42% 19% 1	760
	11th to 20th	Night	18% 4 14% 34% 27% 3	760
	21st to end	Night	19% 4 21% 33% 21% 2	760
October	1st to 10th	Night	23% 5% 23% 32% 17% 1	760
	11th to 20th	Night	26% 6% 25% 33% 11% 1	760
	21st to end	Night	26% 7% 27% 33% 7%	805
November	1st to 10th	Night	26% 7% 23% 31% 12% 1	740
	11th to 20th	Night	24% 9% 30% 26% 11%	760
	21st to end	Night	29% 7% 28% 27% 9%	760
December	1st to 10th	Night	29% 8% 29% 27% 6% 1	760
	11th to 20th	Night	28% 11% 28% 25% 6% 1	760
	21st to end	Night	35% 9% 29% 22% 5%	836

Page generated at 2024-10-21 02:20:03. Version ac07

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